Method for electrolytically coating anodized aluminum with polymers

ABSTRACT

ELECTRICAL COUPLING WITH SAID METAL LENGTH, AND AN ELECTRICAL CONNECTION TO A CATHODE IN CONTACT WITH THE ELECTROLYTE IN THE HYDRAULIC CIRCUIT, SUCH THAT IN THE PRESENCE OF A CONTINUOUSLY SPRAYED LIQUID STREAM OF ELECTROLYTE FROM THE HEAD TO THE METAL, AN ELECTROCHEMICAL CIRCUIT IS COMPLETED VIA THE STREAM FOR CARRYING OUT THE FOREGOING METHOD.   AN ALUMINUM SURFACE IS SPECIALLY ANODIZED TO PROVIDE A POLYMER-ACTIVE OXIDE LAYER, FOLLOWING WHICH A FILM-FORMING POLYMERIC ORGANIC MATERIAL IS APPLIED TO THE TREATED SURFACE AND BONDED TO IT BY MEANS OF THE POLYMER-ACTIVE OXIDE LAYER. THE APPARATUS COMPRISES MEANS FOR CONTINUOUSLY TRANSPORTING A LENGTH OF ALUMINUM METAL IN AND THROUGH A TREATMENT ZONE, A HYDRAULIC CIRCUIT INCLUDING A WETTING HEAD ADJACENT AND GENERALLY DIRECTED AT THE METAL AS IT PASSES THROUGH THE ZONE, THE CIRCUIT INCLUDING MEANS FOR CONTINUOUSLY SUPPLYING ELECTROLYTE TO THE WETTING HEAD TO DISCHARGE THE SAME AT THE PASSING METAL LENGTH, AN ELECTRICSUPPLY MEANS INCLUDING MEANS FOR ANODICALLY EFFECTING AN

March 26, 1974 s Kouc ETA!- 3,799,848

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' MET-HOD FOR ELECTROLYTICALLY COATING ANODIZED ALUMINUM WITH POLYMERS 6 Sheets-Sheet 2 Filed April 1, i971 INVENTORS sow/v 5. ffOL/C BY S/GMU/VD 5525mm p51,; 1% l, ATTORN r:

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Mmh 26, 1974 E- KoLlc T 3,799,848

METHOD FOR ELECTRDLYTICALLY COATING ANODIZED ALUMINUM WITH POLYMERS 6 Sheets-Sheet 5 Filed April 1. i971 s-nw 'sss/vxau-u W711! aarxo INVENTORS EDWIN s. KOL IC BY SIGMU/VD BEREDAY w W i m A TTORA/E/S March 26, 1974 Filed April 1, 1971 E. s. Kouc ET AL 3,799,848

OR ELECTROLYTICALILY COATING ANODIZED ALUMINUM WITH POLYMERS METHOD F 6 Sheets-Sheet 4.

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METHOD FOR ELECTROLYTICALILY COATING ANODIZED ALUMINUM WITH POLYMERS Filed April 1, 1971 6 Sheets-Sheet 5 10,000 I I I %8 4-000 ITO 37 TIMES MIN-C-D.

1.000 P 1 PRE- N FERREP 4- To 400 T IME S MIMC q 5 2 g 00 In Q J k I00 E 80 O g U PREFERRED CURRENT DENSITY FOR \PRODUCING rum CLEAR POLYMER- ACTIVE FILMS 01v AA3003 w 20 VOL. H3 P04 8 x O MINIMUM cunnsur DENSITY FOR 4 X PRODUCING POLYMER-ACTIVE FILMS o/v AA 3003 IN 20 VOL.% H P04 2 M 1. I I 1 1 1 I I 60 I00 no r30 I40 I50 I90 TEMPERATURE F FIG. IO

INVENTORS sow/Iv s KOLIC BY SIGMU/VD BEREOA) much 26, 1.974 KOI-IC METHOD FOR ELECTRQLYMCALLY comma ANODIZED ALUMINUM WITH POLYMERS 6 Sheets-Sheet 6 Filed April 1, i971 mxkuwkwa 5 3x Qw$ X S Q & 3 ww 3 ms mw NQ INVENTORS 5. [(04 IC BY SIGMUND 5E? United States Patent US. Cl. 204-38 E 13 Claims ABSTRACT OF THE DISCLOSURE An aluminum surface is specially anodized to provide a polymer-active oxide layer, following which a film-forming polymeric organic material is applied to the treated surface and bonded to it by means of the polymer-active oxide layer.

The apparatus comprises means for continuously trans porting a length of aluminum metal in and through a treatment zone, a hydraulic circuit including a wetting head adjacent and generally directed at the metal as it passes through the zone, the circuit including means for continuously supplying electrolyte to the wetting head to discharge the same at the passing metal length, an electricsupply means including means for anodically elfecting an electrical coupling with said metal length, and an electrical connection to a cathode in contact with the electrolyte in the hydraulic circuit, such that in the presence of a continuously sprayed liquid stream of electrolyte from the head to the metal, an electrochemical circuit is completed via the stream for carrying out the foregoing method.

This invention relates to the surface treatment of aluminum and, in particular, to a method and apparatus for producing a polymer-active surface thereon and for firmly bonding thereto as a protective coating a film-forming organic material. The invention also relates to products produced by the method.

STATE OF THE ART Many methods have been proposed for bonding filmforming polymers to aluminum surfaces but, generally speaking, the bond between the polymer film and aluminum has not been too satisfactory. A particularly difficult polymer to bond to aluminum is polytetrafluoroethylene (known by the trademark Teflon) One method proposed has been to interpose an intermediate primer layer between the metal surface and the polymer layer. This method requires critical handling of the materials, is rather costly and does not assure a strongly adhering coating. One of the main problems with such methods in which the bond is generally of the physical type is the intrinsic non-adhesive characteristics of polytetrafl'uoroethylene.

A method proposed in US. Pat. No. 2,944,917 comprises providing an aluminum surface having anchoring cavities characterized by undercuts etched therein of average size of less than 30 microns. On the surface is deposited an aqueous dispersion of polytetrafiuoroethylene as a succession of elementary layers, each of the layers being dried and fused before the next layer is deposited. This type of bonding is mechanical in nature.

In still another method (US. Pat. No. 3,304,221), polytetrafluoroethylene is applied to a roughened substrate (e.g. aluminum) in the form of a powder of at least 75 microns in size upon which is then superimposed a continuous film of the same material at a temperature at least about 650 F. to cause the formation of a mechanically bonded laminate.

3,799,848 Patented Mar. 26, 1974 In a method of adhesive bonding, referred to as the Fokker process, an aluminum substrate is prepared for coating by anodizing the aluminum in a chromic acid solution at a temperature of about 104 F. following which an organic adhesive is applied to the surface. The bond obtained compared to the same coating produced on the substrate having a pickled only surface showed that the peel strength of the bond on the anodized surface was substantially the same as the bond obtained with the pickled only surface, except that the bond data obtained using the anodizing treatment showed less scatter and appeared more uniform.

In a recent patent, No. 3,533,920 (Oct. 13, 1970) a method is disclosed of bonding a polymeric fluorocarbon (e.g. polytetrafiuoroethylene) to an aluminum surface previously anodized to provide an oxide film thickness of at least 1 mil (1000 microinches), the bath employed comprising 4% to 6% by volume of 66 Baum sulfuric acid, 0.5 to 6% oxalic acid and 5 to 25 grams per gallon of tannic acid. The aluminum is anodized at a current density of 20 to amps/sq. ft. until a coating of at least 1 mil thick is obtained. The purpose of the thick coating is to provide a. porous layer. Dispersed particles of the fluorocarbon material are then impregnated into the pores by immersion of the anodized aluminum in an aqueous dispersion of the particles so that the particles enter the interstices and the pores of the oxide layer to provide mechanical bonding of the polymer to the aluminum after baking and sintering at an elevated temperature.

Generally, the use of anodized films in the prior art as a base on aluminum for receiving organic coatings has been predicated on employing relatively thick oxide films (up to 1 mil or more) characterized by a relatively high degree of porosity at the surface. It has been reported that, as a general rule, factors which increase the porosity of an anodic oxide film will improve the adhesion of a paint or organic coating. To assure the desired degree of porosity, relatively thick anodic films are produced since it is in the production of such films at fairly long treatment times and/or high current density that porosity is obtained at the surface of the oxide film. Such techniques are economically unattractive due to the cost of forming thick oxide films and, moreover, the oxide films so produced do not always provide good bonding characteristics with film-forming organic polymers.

It would be desirable to provide a method of anodically treating aluminum surfaces with the object of making such surfaces polymer-active to a broad spectrum of polymers without requiring the formation of thick oxide coatings.

OBJECT OF THE INVENTION It is thus an object of the invention to provide an improved method for bonding a film-forming polymer to an aluminum surface.

Another object is to provide a superior polymer-coated aluminum product characterized by enhanced strength of bond, particularly under environmental conditions which tend to attack the unprotected aluminum substrates.

It is a further object of the invention to provide an economic method for treating an aluminum member on a continuous basis, wherein the aluminum is successively cleaned, anodically treated in a special manner and then coated with an adherent layer of a film-forming organic polymer, for example halogen substituted hydrocarbons, such as polyvinylidene chloride, polytetrafluoroethylene, and the like.

1 R. Hovwink and G. Salomon Adhesion and Adhesives, vol. 2, 2nd ed., Elsevier Publishing (10., Amsterdam-London-New York, 1967.

A still further object is to provide an apparatus capable of continuously cleaning and coating aluminum products with a film-forming organic material.

These and other objects will more clearly appear when considered in the light of the following disclosure and the accompanying drawings, wherein:

FIG. 1 is a production-flow diagram schematically showing a succession of steps in carrying out the method of the invention;

FIG. 2 is a simplified view in perspective, and partly broken-away, showing apparatus of the invention at one of the locations of FIG. 1;

FIG. 3 is a sectional View, taken generally at the vertical plane 3-3 of FIG. 2, with partly broken-away elements;

FIG. 4 is a simplified view in perspective to illustrate alternative structure for part of the apparatus of FIG. 2;

FIG. 5 is a sectional view of one of plural elements of FIG. 4, taken generally in the plane 5-5 of FIG. 4;

FIG. 6 is a simplified, fragmentary view in side elevation, to illustrate operation of the apparatus of FIG. 4;

FIG. 7 is a simplified, fragmentary view in perspective, to illustrate a further modification of the invention;

FIG. 8 is a graph showing that the limiting oxide thickness is proportional to the applied current density and that the polymer-active oxide structure is achieved when dissolution of the oxide film occurs;

FIGS. 9 and 10 depict a log-linear plot of current density vs. temperature in degrees F. illustrating the conditions for producing polymer-active oxide structures using anodizing solutions of sulfuric acid and phosphoric acid, respectively; and

FIG. 11 depicts a corrosion-rating chart for evaluating the corrosion resistance of treated aluminum surfaces.

STATEMENT OF THE INVENTION Stating it broadly, the method aspect of the invention comprises, providing a clean aluminum member, electrolytically subjecting the member to anodic treatment in an aqueous anodizing electrolyte maintained at a predetermined temperature to provide oxygen ions at the surface of the member for forming an oxide layer thereon of finite thickness, continuing the anodic treatment at a current density predetermined to form said oxide layer for a time at least sufficient to reach said finite thickness at said predetermined temperature and effect partial dissolution of said oxide layer during the anodic formation of the oxide and thereby produce a polymer-active oxide surface, and then applying a film-forming polymeric organic material to the treated surface.

It is preferred for optimum results that the anodic treatment he carried out for a time at least sufiicient to produce an oxide layer of limiting or maximum film thickness determined by the selected current density and the temperature of the electrolyte, the thickness of the limiting film approximating the ratio of the selected predetermined current density to the minimum current density at which the oxide layer can be formed. The limiting film thickness of a particular current density results when a steady state or substantially an equilibrium is achieved between the formation rate and dissolution rate of the oxide layer, whereby the surface of the oxide becomes polymer-active. Thus, where the selected predetermined current density exceeds the minimum current density, the ratio of the limiting film thickness of the two current densities is greater than one.

The limiting film thickness for a particular current density varies with the electrolyte and the operating conditions, especially the temperature, which affects the dissolution velocity.

An advantage of the foregoing method in producing polymer-active oxide layers is that thick porous oxide layers are not required to promote bonding between a film-forming polymer and an aluminum surface, although thick oxide layers can be used. On the contrary, the polymet-active oxi e ye can. be very th -g. below 250 microinches, or even below microinches, or, advantageously, range up to about 30 microinches and still provide a very strong bond with the applied polymer. Because very thin oxide layers provide good results, very short anodic treatment times can be employed which make the method economically attractive.

, The polymer-active or polymer-receptive oxide structure remaining on the surface is characterized by being electrically conductive.

The foregoing method is particularly applicable to a continuous process and can be mechanized using an apparatus having means for continuously transporting a length of aluminum metal in and through a treatment zone, a hydraulic circuit including a wetting head adjacent and generally directed at the metal as it passes through the zone, the circuit including means for continuously supplying electrolyte to the wetting head to discharge the same at the passing metal length, and electricsupply means, including means for anodically effecting an electrical coupling with said metal length and an electrical connection to a cathode in contact with the electrolyte in the hydraulic circuit, such that in the presence of a continuous sprayed liquid stream of electrolyte from. the head to the metal, an electrochemical circuit is completed via the stream. The electrical coupling with the metal length may be by direct electrical contact or contact through the electrolyte as may be obtained by using a bipolar arrangement of the electrodes.

In the situation where the aluminum member already has an oxide layer prior to treatment, it is cleaned by electrolytically stripping it in situ and the treatment continued at a predetermined current density and temperature to provide preferably a balance between the formation rate and the dissolution rate so as to produce the polymer-active oxide surface.

The principal variables which are considered in carrying out the method of the invention on an aluminum member using an anodizing electrolyte are current density, temperature and time. The electrolyte is preferably selected from the group consisting of phosphoric acid, chromic acid, sulfuric acid, oxalic acid, and mixtures of one or more of these acids, and alkali metal carbonates.

The polymers which produce the desired coatings on aluminum are those referred to as film-forming organic polymers. Examples of such film-forming polymers are:

(1) acrylic resins, such as copolymers of esters of acrylic and methacrylic acids, a specific resin being a copolymer of methyl acrylate and methyl methacrylate;

(2) epoxy resins, such as glycidyl ether of bisphenol A cured with triethylene tetramine;

(3) silicone resins;

(4) halogen substituted hydrocarbons, such as polyvinylidene chloride, polychlorotrifluoroethylene, polytetrafluoroethylene and the like;

(5) polycarbonate resins, such as poly(bisphenol A carbonate);

(6) polyimide resins;

(7) hydrocarbon polymers, such as polypropylene and polyethylene; and

(8) polyurethane resins.

DETAILS OF THE INVENTION In carrying out the preferred method of the invention, the predetermined current density and the temperature of the anodizing electrolyte should be at least sufiicient after a suflicient time of anodic treatment to achieve substantially an equilibrium between the formation rate and the dissolution rate of the oxide film. When this occurs, a limiting or maximum film thickness of the oxide results after a given time period beyond which there is no further increase in the oxide film. At longer treatment periods, after the limiting film thickness has formed, it may tend to decrease in thickness. The expression substantially in equilibrium includes the situation where there is a dropping off in limiting film thickness due to a slight increase in the dissolution rate relative to the formation rate. It should be noted that the limiting film thickness varies with the particular aluminum alloy being anodized, other things being equal.

Generally, prior to anodizing, aluminum is cleaned with 5 an alkaline cleaner which usually contains an inhibitor to protect the surface of the aluminum from chemical attack by the alkaline environment. Thus, the presence of such inhibitors can have a delaying effect on the dissolution of the oxide surface which is being formed, in which case it is important to carry out the anodizing treatment at the predetermined current density and temperature for a time until the limiting film thickness is obtained, as this assures dissolution and subsequent formation of the polymer-active oxide surface. Omitting inhibitors from the cleaning solution facilitates dissolution and subsequent formation of the polymer-active oxide surface when the aluminum member is treated in accordance with the invention.

The importance of working at the limiting thickness of the oxide film in producing the polymer-active oxide surface will be apparent by referring to Example 1 and FIG. 8 which relate the limiting oxide thickness to the applied current density.

EXAMPLE 1 The tests were carried out using a sulfuric acid electrolyte of about 12 vol. percent concentration at 72 and bright-buffed Aluminum Alloy 1100 2 x 6-inch panels. Each panel was pretreated by (1) soak cleaning in a conventional commercial mild alkaline cleaning solution containing inhibitors; (2) water rinsing; (3) dipping in 50 vol. percent HNO solution for seconds; and (4) water rinsing. The panel was placed into, and removed from, the sulfuric acid electrolyte with current on and electrical leads attached. The rectifier was preset to the desired value prior to treatment. The bath was not agitated during the anodic treatment. A Vari Tech model VT 1176A coulometer was used to measure the charge quantity of electricity passed. The oxide film thicknesses were measured on a polished section using a microscope with a 40X objective and a 6X micrometer eyepiece.

Following anodic treatment, the panel was blown dry with air at 78 F. and a specimen was cut out of the center area for oxide thickness measurements. A coating was then applied by dipping the panel into a tetrahydrofuran solution containing 60 g./l. of vinylidene chloride polymer. The coating was air dried at 75-80 F. Coating adherance was based on a ISO-degree bend and a peel test with scotch tape after the panel was exposed to CASS environment for 17 continuous hours. Ratings were assigned as follows: very good, no coating pulled from surface in bent area; good, up to coating pulled from the surface in bent area; fair, 25-50% coating pulled from surface bent area; poor, more than 50% of coating pulled from surface in bent area.

The CASS test is accepted standard procedure for evaluating the corosion resistance of treated aluminum surfaces and the term CASS stands for copper-accelerated acetic acid salt spray test identified as ASTM designation 368-64T.

The procedure employed to evaluate corrosion resistance is similar to the ASTM method detailed in the 1953 Committee Report (Proc. Am. Soc. Testing Mat., 53, 265, 1953). The corrosion performance rating is determined by calculating the percent weighted area which is defective and reading the rating from the conversion plot illustrated in FIG. 11.

In making the evaluation, the percent defective area is determined by comparing test specimens with the data of standard charts depicting unit CASS ratings ranging from 0 to 10. Referring to FIG. 11, it will be noted by way of 70 example that a CASS rating of 7 corresponds to a percent weighted defective area of 0.5, and that a CASS rating of 5 corresponds to a percent defective area of 2.0. On the basis of percent weighted defective area, a CASS rating of 7 indicates a fourfold superiority over a CASS 75 rating of 5. Where longtime eflfectiveness of the coating is to be evaluated, 17-hour CASS ratings are determined, the l7-hour test being a very rigid evaluation.

Each of the aluminum panels was treated at various times in the sulfuric acid electrolyte and the oxide film measured after each of the stated time intervals shown in Table 1 at current densities of l5, l0 and 5 amps/sq. ft, respectively. Each oxide thickness was then coated with vinylidene chloride polymer, as described hereinabove. The results are given in Table 1 as follows.

TABLE 1 Adhesion of vinylidene Treat- Charge Oxide chloride ment Couquantity, thickcoating after Experiment time, lombs, amp ness, 17 hours number min. amp-see. min/it. mils CASS Nominal current density 15 amp/it.

6 540 90 0. 131 Poor. 10 960 160 0. 175 D0. 35 3. 150 525 0. 630 D0. 40 3. 860 643 0. 788 Do. 50 4. 710 785 0. 927 Do. 60 5. 370 895 1. 050 D0. 66 6. 000 1. 000 1. 190 D0. 70 6. 630 1.105 1. 224 Very good 6. 950 1, 160 1. 240 D0. 8. 330 1. 385 1. 225 D0. 108 9. 600 1. 600 1. 208 D0.

Nominal current density 10 amp/it.

30 1. 784 297 0. 350 Poor. 50 2. 905 484 0. 552 D0. 60 3. 524 587 0. 683 Do. 70 4. 108 697 0. 822 Do. 80 4. 717 787 0. 875 Very good 5. 989 1, 000 0. 875 Do. 6. 974 1. 160 0. 856 D0.

Nominal current density 5 a.mp./it.

19 30 844 0.175 Poor.

22 90 2.551 425 0.385 Verygood. 23 120 3, 460 576 0. 350 Do.

It will be noted from Table l and FIG. 8 based on Table 1 that when the limiting oxide film thickness is reached for a particular current density, very good bonding of the polymer vinylidene chloride coating is obtained. However, as for the values falling on the slope of the curves of FIG. 8, it will be noted that the oxide film is not polymer-active and does not become polymeractive until dissolution is achieved at the limiting film thickness. The very good CASS ratings obtained after 17-hour CASS environment were at least about 8 on the CASS rating chart of FIG. 11.

As will be apparent from FIG. 8, the limiting film thickness is dependent upon the current density used, other conditions being equal. Thus, by controlling the current density to any particular value, any desired limiting film thickness can be produced having polymer-active oxide structure or surface. In this connection, the invention is particularly advantageous in that a relatively broad range of conditions is possible over which very thin polymeractive films can be produced without requiring prolonged treatment times. To illustrate the manner in which the ultimate limiting film thickness can be controlled, based on current density and temperature, tests were conducted on Type 3003 aluminum alloy. The current densities at which polymer-active oxide films could be produced were determined by first establishing the logarithmic relationship between the temperature of the electrolyte and the minimum current density at which a polymer active surface is obtained of finite thickness as shown in FIGS. 9 and 10. Thereafter, in determining the optimum or permissible higher current density, the current density was increased from the minimum to that value of current density corresponding to the desired polymer-active limitiny film thickness. Tests were conducted on bright-buifed Type 3003 aluminum alloy panels in 15 vol. percent sulfuric acid and 20 vol. percent phosphoric acid over a rela- 7 tively broad temperature range. The panels were cleaned by: (1) scrubbing the surface thereof with a cotton wad saturated with a slurry of magnesium oxide and water and (2) water rinsing while Wiping the surface with a clean piece of cotton to insure complete removal of any magnesium oxide from the aluminum surface. The minimum current density for producing a polymer-active film of limiting thickness on the cleaned aluminum surface was determined 'by varying the current density at a particular temperature from a lower current density to a high value until a polymer-active oxide film was obtained as characterized by very good adherence obtained with vinylidene chloride after 17-hour CASS, the polymer being applied in the manner described in Example 1. Thus, in FIGS. 9 and 10, a polymer-active surface could not be produced below the lines designated as the minimum current density. The results obtained are given in Tables 2 and 3 as follows.

TABLE 2 Aluminum Alloy 3003 and 15 v/o H2804 Treat- Charge vinylidene Current ment quantity, chloride Temp density, time, amp-minJ Color of adherence, 17- Number F. amp/it. min. it. oxide film hour CASS A. Minimum current density tests 0.45 15.0 6.8 Clear Very good. 0.40 15.0 6.0 d Do. 0.35 15.0 .3 ......do.. Do. 0.30 20.0 6.0 .do Do. 0. 28 20.0 5.6 do- Poor. 0.26 20.0 4.0 -do D0.

3.8 5.0 19.0 -do. Very good. 2.2 7.0 15.4 do Do. 1.5 10.0 15.0 do. Do. 1. 2 10. 12. 0 do Do 1.0 12.0 .0 .....do Do. 0.8 15.0 12.0 .do. Poor. 4.0 5.0 20.0 do Very good. 3. 6. 0 22. 0 -.-do Do 3. 2 8. 0 25. 6 .do Do 3.1 8.0 24.8 .-do. Do 3.0 7.0 21.0 do Poor 2.8 8.0 22.4 do 0 B. Maximum current density 11. 0 8. 0 88 0 Clear- Very good. 39. 0 2. 0 0 ......do Do. 118.0 1.0 118. 0 do D6.

0. Preferred current density for thin films 78=l=1 1 6 15.0 Ve 00d. 78=l=1 1. 1 11. 0 D o. 78:1;1 1. 0 6. 0

78=l=l 0. 9 10. 0 78=l=1 0. 10. 0 78ml 0. 45 20.0 78=l=1 0.42 8. 0 106:1:1 7. 0 2. 5 106=i=1 5.0 3. 5 17.5 --;..do o. 106=t=1 3. 5 1. 8 17. 5 Slightly iri- Do.

descent. 106i1 2. 2 5. 0 1e D0. 106:1:1 1. 9 3. 0 D0. :1:1 31.5 0.8 Poor. 130=i=1 10.5 1.8 19. 0 Slightly iri- Very good.

descent. 13011 10.0 1. 7 17. 0 Clear Do.

1 Steady state current density.

TABLE 3 Aluminium Alloy 3003 and 20 v/0 phosphoric acid vinylidene Treat- Charge chloride Current merit quantity, adherence, Temp., density, time, amp-minJ Color of 17-h0ur Number amp/it. min. it. oxide film CASS A. Minimum current density tests 82=|=1 2. 4 10. 0 82:1:1 2. 2 10. 0 82:1:1 2. 1 10. 0 82:1:1 2. 0 10. 0 97i1 5. 4 4. 0 97=t=1 5. 2 4. 0 97=|=1 5. 0 4. 0 975:1 4. 8 4. 0 9&1 4. 6 5. 0 9711 4. 4 5. 0 127=l=1 28. 0 0. 8 127=|=1 27. 0 0. 8 127=|=1 26. 0 0. 8 127=f=1 25.0 1. 5 127=i=1 24. 0 0. 8 127:i:1 22. 0 0. 9

B. Maximum current density 82:1:1 82 1. 5 123 Clear Very good. 97:1:1 1. 0 1 Do. 1271 920 0. 75 D0.

TABLE 3-Continued Treat- Charge Current ment quantity, Temp, density, time, amp-min] Color of Number F. amp/it. min. ft. oxide film Vinylidene chloride adherence, 17- hour CASS 0. Preferred current density for thin films Irridescent.. Very) good.

1 Steady state current density.

It will be noted from FIGS. 9 (sulfuric acid) and 10 (phosphoric acid) that the current density for producing a polymer-active oxide surface of limiting film thickness varies logarithmically with temperature, the higher the temperature, the higher the minimum current density necessary to produce a polymer-active surface surface of limiting film thickness. For Aluminum Alloy 3003, the minimum in FIG. 9 for sulfuric acid is represented by line AB of the chart, while the desirable working maximum is represented by line DC, the values on line DC for a selected temperature being about 37 times the minimum values along line AB. The preferred range is encompassed by area ABHG, the values along line GH for a selected temperature being about 7 times the minimum represented by line AB. A preferred current density for producing very thin clear polymer-active oxide films on Aluminum Alloy 3003 is that encompassed by area ABFE, the values along line EF for a selected temperature being about 3.5 times the minimum current density represented by line AB. It should be noted that in FIG. 9, the preferred current density is only one-tenth the current density normally used in conventional anodizing at 68 F. and the maximum current density is about one-third.

In the case of FIG. 10 for the phosphoric acid electro lyte, similar results are indicated but over a different range of current densities. The minimum current density values are along line II, the preferred maximum values being along line LK, the values for a selected temperature on line LK being about 37 times greater than corresponding minimum values along line I]. The preferred range is encompassed by area IJPO, the values along line OP for a selected temperature being about 7 times the minimum along line I]. A preferred current density for producing very thin clear polymer-active films on AA 3003 is that encompassed by area II NM, the values along line MN for a selected temperature being about 1.7 times the minimum represented by line I]. The preferred concentration of phosphoric acid regarding the above may range from about 10 to 30 vol. percent.

The minimum current may vary according to the particular aluminum alloy. However, whatever the minimum current density, the operable current density may range broadly up to 37 times the minimum and the preferred current density may range up to 7 times the minimum.

An advantage of working with thin clear polymeractive films is that specularity can be maintained by applying clear film-forming polymers to the surface thereof while obtaining very good adherence.

Mechanism of formation of the polymer-active oxide layer To understand the mechanism of formation of the polymet-active layer, it would be helpful to define the terms limiting or maximum film thickness, oxide formation rate and oxide dissolution rate.

The limiting film thickness is the maximum oxide thickness that can be attained on aluminum during anodizing for a particular applied current density. For all conditions of anodizing, films can reach a maximum limiting thickness at some point in time and may thereafter begin to decrease in thickness. According to some observers, this decrease in film thickness is a result of an increase in film resistance of thicker oxide layers and the generation of heat at the electrolyte-oxide interface which accelerates the dissolution of the oxide film to the extent that the dissolution rate exceeds the rate of formation of the oxide.

The oxide formation rate is the electrochemical rate of growth of the oxide layer at the aluminum-aluminum oxide interface. At a current efiiciency of the oxide formation rate can be calculated by Faradays law of electrolysis which states that 96,500 coulombs (ampereseconds) are required to form one electrochemical equivalent weight of oxide.

With regard to the oxide dissolution rate, this is defined as the rate of dissolution of the oxide film at the oxideelectrolyte interface. This parameter is determined experimentally since it is dependent on several variables.

Thin limiting films can be produced by changing operating conditions as follows.

With regard to the production of polymer-active grade films, it is believed that the oxide dissolution which occurs during limiting film formation, collapses the oxide cells which, while being polarized with an anodic potential, result in the formation of a defective crystal lattice structure having charged centers. The oxide structure obtained is polymer-receptive and electrically conductive.

The electrical conductivity of the polymer-active oxide film, especially thin films of thicknesses ranging up to about 30 microinches, is determined by placing a l-inch diameter polished copper rod weighing 347 grams on the surface and impressing a 20-volt potential across the 'anodic oxide film. A drop of only 40 to 200 millivolts at 650 milliamps has been measured for an oxide film shown to be polymer receptive. For substantially nonconductive films, less than 10 milliamps of current have been observed at an impressed voltage of 20 volts.

The oxide dissolution rate, which is determined experimentally, is affected by the type and concentration of the electrolyte, temperature of the solution, and also whether or not the solution is agitated. Thus, while dissolution rate may vary with change in electrolyte composition, as stated above, this parameter can be determined easily by experiment for each composition depending upon the aluminum alloy being anodized.

As stated hereinbefore, it is preferred in carrying out the invention that the anodizing be eifective to produce the limiting film thickness since the limiting film thickness is not substantially affected by the presence of inhibitors in the anodizing solution.

Since the formation of the polymer-active surface at the limiting film thickness is achieved by providing substantially a steady state or an equilibrium between the formation rate and the dissolution rate of the oxide, it is desirable not to have the dissolution rate during the surface treatment exceed the formation rate to such an extent that there is a complete stripping off of any oxide layer initially formed on the surface. This could occur in a continuous flow system if the temperature and/or agitation at the anode surface are too excessive. Considerable agitation of the solution relative to the surface being treated could, through excessive erosion, prevent any formation of the oxide. Thus, the relative velocity between the solution and the aluminum element should preferably be as close to zero as possible, although this is not essential so long as erosion of the oxide surface is avoided.

As illustrative of various embodiments of the invention, the following additional examples are given.

EXAMPLE 2 Sulfuric acid is an example of a conventional acid anodizing electrolyte when used in concentrations of about to 30 percent by volume, e.g. about to 25% by volume, over current densities of 10 to 25 amps/ sq. ft. at moderately low temperatures of 65 to 80 F. The foregoing conditions in conventional practice generally produce an oxide which in dense and hard. This oxide, however, is not polymer-active, although some mechanical bonding is possible with it.

In producing a polymer-active surface using sulfuric acid, a buffed aluminum panel (2 inches wide by 6 inches long and 0.063 inch thick, identified as Alloy 3003), is precleaned by (1) vapor degreasing in trichloroethylene; (2) soaking in an alkaline cleaning solution (inhibited alkaline cleaner containing basic alkaline salts, surfactants, and emulsifying agents) for 90 seconds at 175 F.; (3) water rinsing for 60 seconds at 120 F.; (4) acid dipping in a solution containing 50 volume percent nitric acid for seconds; and (5) water rinsing for 30 seconds at approximately 78 F. The cleaned panel is then immersed in a 15 volume percent sulfuric acid solution at 130 F. and treated anodically at a current density of 10.5 amps/sq. ft. for 1.8 minutes at a direct voltage of 2.4. Thereafter, the panel is removed, water rinsed at 76 F. and forced-air dried at 78 F. As will be noted from FIG. 9, the current density employed is in the preferred range.

The foregoing treatment produced a polymer-active surface which was also characterized as being substantially electrically conductive.

Following the treatment, the panel with the polymeractive film or layer was dipped in a solution of vinylidene chloride and tetrahydrofuran containing 60 g.p.l. (grams per liter) of the vinylidene chloride for about 3 seconds to provide a 0.2 mil layer coating which was air dried at 78 F. The coated area without baking exhibited very good adhesion when the panel was bent upon itself at 180 degrees and subjected to a peeling test by pulling on the bent edge with adhesive or Scotch tape applied to the surface of the coating. To show that the adhesion 12 was more than a mere mechanical adhesion, the panel was subjected to a CASS test for 17 hours, after which the bend and peel tests were conducted on other portions of the panel. The panel, following the CASS test, exhibited very good adhesion and the CASS rating was at least about 8.

Tests have shown that the behavior of electrolytes are affected by various parameters. For example, electrolytes behave differently from each other at various temperatures and current densities. For instance, sulfuric acid provides very good adhesion after the CASS test where the polymer-active layer is produced at 168 F. at a treatment time of 7.5 minutes and a current density of 60 amps/sq. ft. (at 188 amp-sec./sq. inch or 450 ampmin./sq. ft.). However, when the aluminum panel was treated at F. for the same time of treatment (7.5 minutes) at a current density of 60 amps/sq. ft. (450 amp-min./ sq. ft.), the adherence of the vinylidene chloride coating was poor after the CASS test, although the adherence appearer to be very good before the CASS test. By dropping the temperature down to 120 F., the treatment time was not suflicient to produce the limiting film thickness and, with it, the polymer-active surface. However, the treatment time of 7.5 minutes was sufiicient at 168 F.

Phosphoric acid appears to give good results over a relatively broad range of temperatures, e.g. 61 to 110 F. and higher, and over a range of 5 to more than 20 volume percent. A 15 volume percent solution of phosphoric acid gave excellent results on Alloy 3003 (cleaned as in Example 1) panel when coated with vinylidene chloride both before and after a 17-hour CASS test.

An electrolyte solution containing 3.8% by weight of sodium carbonate gave very good results at a temperature of 104 F. and 18 amps/sq. ft (450 amp-min./sq. ft.) with vinylidene chloride (25 minutes treatment time). However, poor results were obtained at 81 F. and 10 amps/sq. ft. at 45 minutes of treatment time (450 ampmin./sq. ft.), both before and after the CASS test due to the fact that the time of treatment was not sufiicient to form the limiting film thickness and, with it, the polymeractive surface.

In the case of an electrolyte solution containing 10% by weight of oxalic acid, very good adhesion of vinylidene chloride was obtained at a temperature ranging from about to 141 F., a current density of about 27 to 34 amps/sq. ft. (450 amp-min./sq. ft.) while poor adhesion was obtained at 50 amps/sq. ft. (450 amp-min./ sq. ft.) at 77 F. and 35 amps /ft. (450 amp-minJft?) at 115 F. temperature.

Similarly, very good results have been obtained with chromic acid solutions (e.g. 5% by weight) at temperatures of F. to F., at 38 to 70 amps/sq. ft. (450 amp-min./ sq. ft.) before and after a 17-hour CASS test with vinylidene chloride, while poor results were obtained at 4 amps/ft. (450 amp-min./ft. at 80 F.

The foregoing poor results were due to the fact that the time of anodizing was not sufficient to reach limiting film thickness and the accompanying polymer-active surface.

As short time treatments and thin films are particularly desirable in carrying out a continuous process commercially, an electrolyte containing phosphoric acid is preferred.

EXAMPLE 3 As stated hereinbefore, when the dissolution rate of the aluminum oxide exceeds the formation rate, a polymer-active oxide surface is not obtained. This is illustrated by the following.

A buffed 2 x 6 inch aluminum panel (Alloy 3003) was cleaned as described in Example 2 and thereafter treated anodically in a 20 volume percent solution of phosphoric acid electrolyte at 127 F. and 24 amps/sq. ft. for 0.8

13 minute. A vinylidene chloride coating was then applied as in Example 2, the panel being similarly evaluated before and after the 17-hour CASS test. Due to a rather high dissolution rate, a polymer-active surface was not obtained, as a result of which the applied vinylidene 14 EXAMPLE 5 The purpose of this example is to illustrate the broad spectrum of polymeric materials which can be adherently bonded to aluminum in accordance with the invention.

hl h d b b d f 5 Aluminum panels (Alloy 3003) of 2 x6 inch in size were c on e coating 5 Owe Poor on mg zefore an ter cleaned as in Example 2 and anodically treated in 15 the CASS test. However, at 28 amps/ft. for 0.8 mmute, volume percent phosphoric acid Solution for 5 minutes very good adherence was obtained after l7-hour CASS. at 110 and a current density f 1 amps/L ft to Similarly, the Same l y s tested at 2 F., 16 form an oxide of limiting thickness characterized by the being anodized in 20% phosphoric acid for 10 minutes, 10 polymer-active surface. The materials tested and the reat 2 amps/fe the other for 10 minutes at 2.4 amps/ft sults obtained are given in Tables 4 and 5 as follows.

TABLE 4 Polymer (ZODCGIP Test tration, Application No Polymer Solvent w/o method Drying conditions Polylvinylidene chloride".-- Tetrlhydrofuran do Aqueous dispersion 0 F. 4A Polyvinylidene fiuoride Dimethylformamide Air dg'ile d 78 F., baked 1 hr.

- 02 A Epoxy resin MIBK/xylene/toluene 27 d0 Baked 1.5 hr. 212 F. 6A Silicone resin Toluene/IPA/xylene 12.5 do Air dried 78 F.

Acrylic resin-. Toluene .(1 Baked 1 hr. 149 F.

dn dn Air dried 78 F. 9A Polyurethane Tetrahydrot'uran Baked 1 hr. 149 F. l0A do Air dried 78 F. 11A do Cellosolve acetate/tolueneL 18. 6 Baked 1 hr. 212 F 12A Polycarbonate Methylene chloride do Baked 1 hr. 149 F 13A do .do Air dried 78 F. 14A Tetrafiuoroethylene Aqueous dispersion 30 ...do Baked 15 min. 900 F. 15A Polyimide Dimethyliormamide do Baked 3? 115%}. 300 F. plus 2 min. 00

1 MIBK designates methylisobutyl ketone.

The latter had a polymer-active surface as evidenced by very good adherence of vinylidene chloride after 17-l1our CASS. The one with the lower current density had an excessive dissolution rate and did not produce an adherent coating.

The data of Tables 2 and 3 illustrate how the electrolyte ssytem of, for example, sulfuric acid or phosphoric acid, can be handled by adjusting current density and temperature to avoid an excessive dissolution rate and assure reaching the limiting film thickness at which substantially a balance between the formation rate and the dissolution rate occurs in forming the polymer-active surface.

EXAMPLE 4 This example illustrates how a conventionally anodized aluminum member can be converted to a polymer-active surface.

In converting the surface of a conventionally oxidized member to one which is polymer-active, a buffed 4 x 6 aluminum panel (Alloy 5557) was cleaned as described in Example 2 and conventionally anodized in a 15 volume percent sulfuric acid electrolyte for 15 minutes at 16 amps/sq. ft. and 75 F. This treatment produced a dense hard oxide layer of high specularity. After water rinsing and forced air drying at 78 F., the panel was cut to provide two specimens, 2 x 6 inches in size. One of the specimens with the hard dense oxide layer was coated with vinylidene chloride as in Example 2, but the coating did not adhere and could be easily peeled off.

The other specimen with the hard oxide layer was treated anodically in a 15 volume percent phosphoric acid solution for 2 minutes at 20 amps/sq. ft. at a temperature of 110 F., and its oxide modified by partial dissolution to produce a polymer-active surface. After water rinsing and forced-air drying the panel at 78 F., the vinylidene chloride coating was applied as in Example 2. The polymer coating exhibited excellent adhesion before and after a l7-hour exposure CASS test.

A polymer coating for the purposes of this invention is generally considered good when it exhibits a 17-hour CASS rating of at least 7.

TABLE 5 Comparative surface bonding characteristics-surface prepared as practiced in the invention 1 Adhesion Adhesion after prior to CASS 56 hours CASS exposure exposure Polymer thickness, mil

.- Very good.

1 Aluminum Alloy 3003 panels, 2 x 6 inches in size, cleaned as noted in Example 1 and anodically treated using 15 volume percent phosphori acid electrolyte for 5 minutes at F. and a current density of 1 amps/sq. ft:

The results of Tables 4 and 5 demonstrate the allaround capability of the polymer-active surface produced by the invention of providing excellent adherence with a broad spectrum of polymers even after 56 hours of CASS testing. It will be noted that even tetrafiuoroethylene, the most diflicnlt polymeric material to bond to a sub strate, exhibited excellent adherence to the polymer-active surface.

EXAMPLE 6 The object of this example is to demonstrate that the invention is applicable to a broad range of aluminum alloy products.

Bulfed panels of a wide range of alloys shown in Table 6 were cleaned as in Example 2 and then anodicallvtreated in 15 volume percent phosphoric acid at 110 1'. and Z0 amps/sq. ft. for about 5 minutes, Thereafter, a vinylidene chloride coating was applied in the manner described in Example 2 and each of the coated panels evaluated after 17 hours exposure to the CASS environment. Very good adhesion was observed for all of the alloys tested.

TABLE 6.-SURFACE BONDING CHARACTERISTICS OF TREATED PURE ALUMINUM AND ALUMINUM ALLOYS 1 Panels were cleaned and acid dipped as noted in Example 2 prlor to anodic treatment for 5 minutes in 15 v/o phosphoric acid solution at 110 F. and at a current density of 20 amp/sq. it.

I Panels were coated as noted in Example 2.

EXAMPLE 7 Tests have indicated that the polymer-active surface is heat stable at rather high temperature without substantially losing its characteristic. A 4 x 6 inch panel (Alloy 3003) was pro-treated as in Example 2 and then provided with a polymer-active surface by anodically treating the panel in a 15 volume percent phosphoric acid solution at 110 F. and 16 amps/sq. ft. for 5 minutes. After water rinsing and forced-air drying at 78 F., the polymer-active surface was tested for electrical conductivity by placing the end face of a 1 inch diameter polished copper rod weighing 347 grams on the surface and impressing a 20 volt potential across the polymer-active surface layer. A 70 millivolt drop was measured at 650 milliamps, thus indicating that the polymer-active surface was substantially electrically conductive.

Thereafter, the panel was heated to 1000 F. in air for 2 hours. After cooling, it was found that the electrical conductivity was unchanged. The panel was then coated with vinylidene chloride as in Example 2 and exhibited excellent adherence to the aluminum substrate after 17 hours of CASS testing.

A cleaning solution and method found particularly preferable in preparing the surface for electrolytic anodizing to produce the polymer-active structure or surface are as follows.

The panel which may or may not be vapor degreased is dipped in a solution containing the following ingredients.

The panel is dipped in the above solution for 1 to 3 minutes at 170 F. to 175 F. The panel is thereafter rinsed for about 60 seconds at 120 F. and then subjected to the following anodic treatment in, for example, 20 volume percent phosphoric acid solution. The preferred current density is just below that which produces an irridescent oxide film with a charge quantity of about 20 ampere-minutes/ft. The electrolyte is maintained at 110 F. and the time of treatment is about 15 to 30 seconds. Following the anodizing treatment, the panel is removed, water rinsed at about 78 F. and forced-air dried at about 78 F. preliminary to applying a polymer coating.

Another method of cleaning the aluminum panel is to employ a slurry of magnesium oxide. As illustrative of this method in the production of polytetrafluoroethylene coating (Teflon) the following example is given.

16 EXAMPLE -8 A bright-buffed aluminum panel 2 inches wide by 6 inches long and 0.065 inch thick (AA 1100) is cleaned by: 1) scrubbing the surface thereof with a cotton wad saturated with a slurry of magnesium oxide and water and (2) water rinsing while wiping the surface with a clean piece of cotton to insure complete removal of any magnesium oxide from the aluminum surface. Following cleaning, the panel is treated anodically in a 12 volume cleaning, the panel is treated anodically in a 12 volume percent sulfuric acid solution at about 2 amps/ ft. for a period of 10 minutes (20 amp-min./ft. the temperature of the electrolyte being maintained at about F. This treatment is calculated to give a polymeractive oxide film of about 20 micro-inches in thickness. After the anodic treatment, the panel is removed, water rinsed and forced-air dried at 7 8 F. The polytetrafluoroethylene coating is applied to the surface by (1) dipping the treated panel in a 45% tetrafluoroethylene aqueous dispersion for about 15 seconds and slowly withdrawing to provide a thin wet film, (2) drying the film at F. in air, (3) baking the dried film at 375F. for 3 minutes to volatilize any of the wetting agents present, and (4) then fusing the film by baking at 730 F. for 3 minutes to provide a strongly bonded coating of Teflon.

The adhesion of the coating is tested by scoring the coated surface with a series of crossed lines 0.047 inch apart running through the thickness of the coating and then pulling on the scored area following the application of Scotch tape thereto. The coating does not pull away which demonstrates that it is strongly bonded to the aluminum surface via the polymer-active oxide surface.

As examples of electrolyte acid with compositions (aqueous), the following are given:

(A) phosphoric acid: 5 W0 to 30 v/o; e.g. 10 v/o to (B) sulfuric acid: 5 v/o to 30 W0; eg 10 We to 25 v/o;

(C) oxalic acid: 1% by weight to saturation by weight;

(D) chromic acid: 1% by weight to 25% by weight.

With regard to the foregoing electrolytes, generally speaking, the higher the temperature, the greater is the current density required to effect formation of the polymer-active oxide structures and the shorter the time of treatment. On any event, for a particular alloy, the time employed for a particular electrolyte, composition, etc., should be sufiicient to effect a partial dissolution of the anodic oxide.

The invention is readily adaptable to a continuous process whereby the solution or the member being treated move relative to one another by employing an apparatus system for carrying out two operations in situ, to wit: (1) removing the natural oxide layer from the aluminum member and thereby providing a clean aluminum surface; and (2) forming a layer of the polymer-active oxide. The continuous process is advantageous in that a controlled balance can be achieved and maintained by selecting the current density corresponding to a thin film that is polymer-active in minimum time.

An illustrative embodiment of the method and apparatus of the invention is shown in general outline in FIG. 1, for the continuous mass-production treatment of aluminum rod, strip, or sheet material 10, moving from left to right. The above-noted stripping and oxidizing steps take place at a single station 11, after which the thus-oxidized aluminum is water rinsed at 12 and air-dried at 13, prior to polymer-application at 14 and oven-treatment (if needed) at 15. In general, the aluminum processing at 11 comprises a continuously circulating electrolyte system involving a flow-discharge element or spray-head electrode 16, positioned above the aluminum 10 and flooding the same with the electrolyte; a collector 17 receives the electrolyte after contact with the aluminum 10, and dashed line 18, including a pump 19, will be understood to suggest completion of the circulating system. Preferably,

1 7 the electrode 16 is excited as the cathode, and the aluminum 10 is the anode, as suggested by polarity legends at 16 and at an aluminum-contacting shoe element 20. The aluminum member may be stationary and the electrolyte directed to it at a relatively high rate of flow.

Processing means 11 is shown in greater detail in FIGS. 2 and 3, wherein aluminum bar material 10 is shown being continuously advanced in the right-to-left direction. At its passage through means 11, the aluminum 10 is guided on spaced supporting rolls 21-22 which may be continuously driven by means 23. The rolls 21-22 are fixedly journalled (by means not shown) and are preferably sized for constant partial immersion in the electrolyte, at normal electrolyte level, in the collector 17. Collector 17 may be a rectangular pan, having a central drain 24 which includes orifice means 25 whereby a normal desired electrolyte level 26 can be maintained in pan 17; a filter 27 cleans the electrolyte before it returns to a reservoir or sump 28, from which the pump 19 recirculates clean electrolyte.

The spray electrode 16 is shown in FIG. 3 as a thin and generally rectangular and substantially closed box, having a width W overlapping and extending beyond the e'lfective width dimension W of the aluminum 10 to be treated; its length dimension L (FIG. 2) extends substantially along the path of movement of the aluminum 10 and is shown centrally positioned between rolls 21-22. The bottom surface 29 of head 16 is foraminous or otherwise perforated, the effectively open area of surface 29 being preferably such, in relation to the flow rate and pressure of electrolyte supplied by pump 19, that continuous streams (rather than droplets) of electrolyte span the relatively short space to the top surface of aluminum 10, and such that these streams impinge continuously over and thus flood the entire exposed top surface of the aluminum 10. The head 16 may include an upstanding supply pipe 30, guided for vertical positioning adjustment (suggested by rack-and-pinion means 31) and forming part of the supply circuit 18, through a flexible connection 32. A flexible wiper member 33, fixed to the exit end of head 16, serves to wipe most of the electrolyte from the aluminum 10 prior to passage beyond the collecting area of pan 17. In order to maintain a desired electrolyte temperature condition, at discharge to the aluminum 10, heater means 34 is shown at reservoir 28, supplied by means 35 having a control provision 36 which responds to the instantaneous output of a heat detector or probe 37 (such as a thermocouple, bead thermistor, or the like) carried by and exposed to the flooded interior of head 16. The rectangular prismatic phantom outline 38 over the pan 17 and over the spray electrode 16 will be understood to suggest splash limiting means which serves the further function of a venting hood, with exhaust means 39.

In use, the pump 19 establishes plural liquid streams of electrolyte between head 16 and the aluminum, thus completing the electrical circuit for anodic treatment of the aluminum via aluminum-contacting shoe 20. The number of these streams and the flow rate assure full-area liquid coverage of the top surface of the aluminum, for substantially the full extent of passage beneath head 16. The bottom surface of the aluminum will have been prewetted (with electrolyte from pan 17) by the time the aluminum is exposed to the spray discharge; thus electrolyte that impinges on the top surface of the aluminum and flows over the side edges thereof will be automatically drawn underneath the aluminum, assuring full coverage of the aluminum during its exposure to head 16.

In a typical and successful employment of the invention, it need not matter whether the aluminum to be treated is coated with a naturally formed aluminum oxide or whether the specimen is of a commercially available anodized variety. It also need not matter whether the specimen is or is not chemically clean; for example, a grease-laden specimen is very quickly stripped and cleaned so that the desired treatment takes place nonetheless. The electrolyte may comprise 5 to 30 volume percent solution of phosphoric acid. A bath containing 20 volume percent phosphoric acid and 4.5% by weight of oxalic acid has been found useful in producing polymer-coated aluminum products.

For the short exposure time stated hereinabove, the substrate thickness of the polymer-active layer may be in the order of about 0.001 mil (1 microinch) in thickness and range up to as high as 0.15 or 0.25 mil (150 to 250 microinches) and, more preferably, up to about 0.03 mil (30 microinches).

In the embodiment of FIGS. 4 to 6, the spray electrode 16 is replaced by an array of spaced, parallel, porous roller electrodes 41-42-43-44-45, internally supplied with electrolyte from manifold means 40 connected to supply 18, and having continuous wetting contact with the top surface of the aluminum specimen 10. These rollers are shown to comprise an inner non-rotatable tubular support element 46 (FIG. 5) fixed to its electrolyte supply pipe 46' and extending, in concentrically supporting relation within substantially the full axial length of a porous outer tubular roller element 47; the porous member 47 may be a perforated rigid or semi-rigid basket with a cloth cover of material inert to the electrolyte. Suitable bearing means (not shown) supports the outer element for rotation about inner element 46, and a sprocket wheel 48 on one end of element 47 enables its driven rotation. Sprocket-chain means 49 is shown driving all sprocket wheels in unison, at a speed synchronized with the feed of the aluminum 10 so as to establish noslip contact or near-contact of the wetting means 47 with the top surface of the passing aluminum specimen. A cap 50 closes the undriven end of each roller 41, and this cap may be carried by either of the tubular members 46-47.

As shown in FIG. 5, the cylindrical body of the inner member 46 is locally apertured, as by slits or a window 51, of limited angular extent which may in generaly be up to about Such opening localizes the radially outward flow of electrolyte so as to flood the rotated outer porous member 47 just prior to its contact or near-contact relation with the aluminum 10, such that the velocity of the solution relative to member 10 is as close as possible to zero so as to minimize erosion of the oxide layer. In operation, FIG. 6 shows that a solid wall 52 of liquid electrolyte is established by each roll 41, 42 45 in flooding intimate contact with the entire top-surface extent of the passing aluminum strip 10; this electrolyte spills over the edges of the aluminum, and a substantial fraction thereof runs over the prewetted bottom surface of the aluminum. If desired, bottom rolls 55-56, 57-58- 59, generally similar to rolls 41-42-43-44-45, may be provided for simular contact and Wetting action with the bottom surface of the aluminum. Such bottom rolls should preferably be driven in the opposite direction of the upper rolls, for similar no-slip relation with the passing aluminum specimen, as suggested by arrows. The bottom rolls may also be internally supplied with fresh electrolyte (with internal radial-flow local flooding aperture 51 in the region just prior to contact with the aluminum), but we prefer to rely on the wetting action achieveable through partial immersion of the bottom rolls in the collector-pan electrolyte, as shown in FIG. 6; in the latter event, there is, of course, no need to internally supply the lower rolls. Electrical contact is shown at shoe 20 to the aluminum and at 53 to the manifold which supplies electrolyte to the upper roller-head assembly.

The arrangement of FIG. 7 illustrates application of the invention to treatment of aluminum material of contoured section, as for example the L-shape section or angle of continuous element 10. The material 10' is processed by upper and lower electrolyte-wetting rolls 60-61. The upper roll 60 is externally characterized by a convex surface of revolution to match, in contacting or near-contacting relation, the concave surfaces of the aluminum as it passes the treat ment region. In like fashion, the lower roll 61 is apparently causes polymer films to undergo structural externally characterized by a concave or V surface of modification at the interface and the microscopically revolution to match the convex surfaces of the passing etched nature of the oxide surface. Thus, at the polymeraluminum specimen. Both rolls 60-61 may be of the inaluminum oxide interface, the polymer is less crystalline ternally flooded nature described in conjunction with due to a disordering of the polymer. It is believed that FIGS. 4 to 6; however, in the form shown, a wetting this relaxed conditions causes an ion-dipole interaction head on cathode 62 having an apertured bottom concave or the existence of like forces of similar magnitude apsurface delivers a continuous flow of electrolyte to the proximating chemical bonds in energy.

upper roll 60. Both rolls 60-61 may be cloth-covered, Whatever the theory, the invention provides a superior for maximum entrainment of the electrolyte and to as- 10 polymer-coated aluminum member comprising a filmsure a solid wall of liquid from cathode 62 to anode 10. forming polymeric organic material bonded to the sur- In carrying out the continuous process, an apparatus face of the aluminum member via a polymer-active similar to that shown in FIGS. 1 to 3 was employed in aluminum oxide layer integral with said surface, the polywhich an extrusion of Aluminum Alloy 6063 of 1.75 mer-active oxide layer being further characterized by being electrically conductive.

Although the present invention has been described in inches wide and 0.125 inch thick was used. The aluminum extrusion or member was made anodic and the electrolyte flow was adjusted to 7 liters/minute at a DC. conjunction with preferred embodiments, it is to be unapplied voltage of 30, the aluminum member (4 feet derstood that modifications and variations may be long) being moved at a rate not exceeding 3 feet per resorted to without departing from the spirit and scope of the invention as those skilled in the art with readily understand. Such modifications and variations are conminute. Each-extrusion was pulled through the reaction zone at the desired rate, with the temperature of the sprayed electrolyte controlled at about 122 F. and the sidered to be within the purview and scope of the invencurrent density ranging from about 12 to 27.5 amps/sq. tion and the appended claims.

ft. at retention times of 10 to 60 seconds. The treated ex- Wh i l i d i trusions were thereafter cold water rinsed at 75 'F. and 1 A h d f b di a fi1 -f i Polymeric forced-air d d at Th6 P y bonding charac organic material to an aluminum member which comteristics were evaluated with a vinylidene chloride coating prises,

which was applied to the test pieces by dipping them in a providing an aluminum member with a substantially 60 g.p.l. of polymer contained in a tetrahydrofuran soluclean Surface,

1 .22.23.1 2? reassessed,tarsus: a; mm to tested before and after 17 hours of CASS by blending the ment m an F P anodlzmg Solutlon Selefled from extrusion 90 (which caused the metal to crack) and then the group conslstmg of abfmt 5 P rcent to applying and pulling off Scotch tape from the bend zone P Phosphoflc acld, b ut 5 VOL (peel test). The results obtained are summarized in Table Pare/311t to 30 Percfillt Sulfllflc field, an 7 as follows. oxalic acid solution ranging by weight from about BLE 7.SUM1VIARY OF EXPERIMENTAL WORK ILLUST RATING FEASIBILITY OF SIMULTANEOUSLY TA CLEANING AND PRETREATING ALUMINUM MEMBERS TO PROVIDE A POLYME R-ACTIVE LAYE R ON A CONTINUOUS BASIS Extrusion Bonding characteristics based on vinylidene chloride coating 2 di 1; t condit' ns E A110 0 reatmen 10 Retensilent Adhesion prior to CASS Adhesion after 17 hrs. CASS Current tion ieed exposure 3 exposure 3 Temp., density, time, rate, Experiment number F. amp/it. sec. ftJmm. Top Bottom Top B tto 20 We phosphoric acid and 4.5 w/o oxalic acid 122 27. 5 30 1 Very good Very good.-. Very good Very good. 122 27.5 20 1.5 J D0. 122 27. 5 15 2. 0 J Do. 122 27. 5 10 3. 0 Poor Poor- Poor. Poor. 122 27. 5 15 2. 0 Very good.-.- Very good Very good... Very good. 122 27. 5 1O 3. 0 Poor- P0012. Poor- Poor.

1 Extrusions, 1.75-inch wide by 0.125-inch thick, were not cleaned prior to treating.

2 Six-inch lengths were coated as noted in Example 2.

a Coating adhesion was based on a 90-degree bend and peel test witnScotch Tape.

4 Four-foot extrusions were pulled through the reaction zone at the indicated feed rate to simulate a continuous feed system.

The aluminum extrusions which were treated in ac- 1% to saturation, and (4) a chromic acid solution cordance with the invention contained natural oxide with ranging by weight from about 1% to 25%, grease spots and fingerprints thereon which were visible said anodic treatment being carried out at a preprior to the treatment. The original areas of surface condetermined temperature and a predetermined tamination were marked with a scriber. Following treatcurrent density eifective to form an oxide layer, ment, the natural oxide had been completely removed said predetermined temperature and current along with the grease spots and fingerprints. density being correlated to provide an oxide Under comparable conditions, the minimum treatment layer having a limiting film thickness ranging time required to provide a polymer-active surface was up to about 250 microinches, about the same for stationary or continuously moving carrying out said anodic treatment at said current extrusions. density for a time sufiicient to reach said limiting Phosphoric-oxalic acid electrolyte exhibited good thickness ranging up to said about 250 microinches, throwing powder and it is expected that the electrolyte said limiting thickness being obtained when subwill be capable of providing a polymer-active surface on stantially an equilibrium is achieved between very complex shapes. As can be seen in experiments 16A the formation rate and dissolution rate of said through 21A, a polymer-active surface was obtained on oxide layer at said predetermined temperature the underside of the extrusions without the aid of a and current density, thereby resulting in a polycathode on that side. mer-active oxide at the surface thereof,

It is believed, according to infrared analysis, that the and then applying to said polymer-active oxide layer excellent adhesion obtained with the variety of different a film-forming polymeric organic compound selected polymeric materials appears to be due to the combined from the group consisting of acrylic resins, epoxy effect of the crystallographic structure of the oxide, which resins, silicone resins, halogen substituted hydro.

21 carbon resins, polycarbonate resins, polyimide resins and polyurethane resins.

2. The method of claim 1, wherein the temperature and current density are correlated to provide an oxide layer having a limiting film thickness of up to about 100 microinches.

3. The method of claim 1, wherein said aqueous anodizing solution is phosphoric acid maintained at a predetermined temperature in the range of about 65 F. to 175 F.

4. The method of claim 1, wherein said aqueous anodizing solution is sulfuric acid maintained at a predetermined temperature in the range of about 65 F. to 175 F.

5. A method of bonding a fihn-forming polymeric organic material to an aluminum member which comprises,

providing an aluminum member having an oxide layer on the substrate thereof,

making said aluminum member an anode in an aqueous anodizing electrolyte and substantially electrochemically removing said oxide layer from the substrate and thereby provide a clean aluminum surface, electrolytically subjecting the clean surface of said member to anodic treatment in said aqueuos anodizing electrolyte selected from the group consisting of (1) about vol. percent to 30 vol. percent phosphoric acid, (2) about 5 vol. percent to 30 vol. percent sulfuric acid, (3) an oxalic acid solution ranging by weight from about 1% to saturation, and (4) a chromic acid solution ranging by weight from about 1% to 25%,

said anodic treatment being carried out at a predetermined temperature and a predetermined current density effective to form an oxide layer, said predetermined temperature and current density being correlated to provide an oxide layer having a limiting film thickness ranging up to about 250 micro-inches, carrying out said anodic treatment at said current density for a time sufficient to reach said limiting thickness ranging up to said about 250 microinches,

said limiting thickness being obtained when substantially an equilibrium is achieved between the formation rate and dissolution rate of said oxide layer at said predetermined temperature and current density, thereby resulting in a polymeractive oxide at the surface thereof,

and then applying to said polymer-active oxide layer a film-forming polymeric organic compound selected from the group consisting of acrylic resins, epoxy resins, silicone resins, halogen substituted hydrocarbon resins, polycarbonate resins, polyimide resins and polyurethane resins.

6. The method of claim 5, wherein the temperature and current density are correlated to provide an oxide layer having a limiting film thickness ranging up to about 100 microinches.

7. The method of claim 5, wherein the aluminum member being treated is substantially stationary and wherein the electrolyte is applied at a relatively high rate of flow to said member.

8. The method of claim 5, wherein said aqueous anodizing solution is phosphoric acid maintained at a predetermined temperature in the range of about 65 F. to 175 F.

9. The method of claim 5, wherein said aqueous solution is sulfuric acid maintained at a predetermined temperature in the range of about 65 F. to 175 F.

10. A continuous method of bonding a film-forming polymeric organic material to an aluminum member having an adherent surface layer of aluminum oxide comprising subjecting said surface to the steps of continuously wetting said member with an aqueous anodizing solution capable of removing said oxide coating,

subjecting said member while in contact with said anodizing solution to an electrical current density at the surface thereof suflicient to remove said oxide coating and provide a clean surface,

continuing said treatment by electrolytically subjecting said clean surface to anodic treatment in said aqueous anodizing solution selected from the group consisting of (1) about 5 vol. percent to 30 vol. percent phosphosphoric acid, (2) about 5 vol. percent to 30 vol. percent sulfuric acid, (3) an oxalic acid solution ranging by weight from about 1% to saturation, (4) a chromic acid solution ranging by weight from about 1% to 25%,

said anodic treatment being carried out at a predetermined temperature and a predetermined current density effective to form an oxide layer, said predetermined temperature and current density being correlated to provide an oxide layer having a limiting film thickness ranging up to about 250 microinches, carrying out said anodic treatment at said current density for a time sufficient to reach said limiting thickness ranging up to said about 250 microinches, said limiting thickness being obtained when substantially an equilibrium is achieved between the formation rate and dissolution rate of said oxide layer at said predetermined temperature and current density, thereby resulting in a polymeractive oxide at the surface thereof,

and then applying to said polymer-active oxide layer a film-forming polymeric organic compound selected from the group consisting of acrylic resins, epoxy resins, silicone resins, halogen substituted hydrocarbon resins, polycarbonate resins, polyimide resins and polyurethane resins.

11. The method of claim 10, wherein the temperature and current density are correlated to provide an oxide layer having a limiting rfilm thickness ranging up to about microinches.

12. The method of claim 10, wherein said aqueuos anodizing solution is phosphoric acid maintained at a predetermined temperature in the range of about 65 F. to F.

13. The method of claim 10, wherein said aqueous solution is sulfuric acid maintained at a predetermined temperature in the range of about 65 F. to 175 F.

References Cited UNITED STATES PATENTS 3,672,972 6/ 1972 Dorsey 204-58 3,279,936 l0/1966 Forester 20438 E 2,647,079 7/ 1953 Burnham 204-38 A FOREIGN PATENTS 700,516 12/ 1953 Great Britain 204-38 E OTHER REFERENCES The Surface Treatment and Finishing of Aluminum and Its Alloys, Wernick et al., 1964, pp. 269-270, 345- 348, 357, 365.

JOHN MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. X.R. 204-38 A, 58 

